Not everyone wants a website to handle its theme the same way.
Sometimes you want it to match your device. Sometimes you want it to stay dark all day. And sometimes you'd rather it switch automatically without changing your system settings.
The theme switcher now gives you four choices.
Light and dark always stay the way you choose. System follows your device settings. The new auto mode follows the time of day, using the light theme during the day and the dark theme in the evening.
Your choice is remembered between visits, so once you've picked a mode, it'll be there the next time you come back.
You'll find the theme switcher in the header, next to News. Try each mode and choose the one that works best for you.